A small programming language designed to teach the basics of language implementation.
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Koi is a small programming language that is designed to be easy to use, easy to understand and easy to implement.

Koi is an imperative, dynamic, weakly-typed language with first-class functions. Koi's syntax and features were influenced by JavaScript, Lua and Ruby. Koi makes a point of working very hard to be unambiguous in it's syntax so that it is easy to write parsers for.

Koi's main goal is to be a useful language that teaches the basics of language implementation. The project was started because I wanted to learn more about how languages work 'under the hood' and Koi is my way of sharing what I've learnt with as many people as possible.


This is an old-school 'Blast Off!' program written in Koi:

countdown = function( count )
  print( to_string( count ))
  print( ", " )
  if( count == 0 )
  count = count - 1
  call( countdown, count )

call( countdown, 10 )

print( "Blast Off!" )

#=> 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0, Blast Off!


For ease of use the default installation of Koi is packaged as a RubyGem. Providing you already have Ruby and RubyGems installing Koi is as easy as entering the following command in a terminal:

gem install koi-lang

Mac OS X and most Unix/Linux distributions come with an installation of Ruby and RubyGems. If you do not have Ruby and RubyGems installed please check the Ruby website for instructions.


After Koi is installed you can run a program by typing the following on the command line:

koi /path/to/program

Koi Fundamentals

Data types

Koi features a full set of basic and compound data types including:

  • Nil

  • Boolean

  • Integer

  • Float

  • String

  • Hash

  • Function

Flow control

Koi currently only implements a minimal set of flow control operators:

if( expression )

unless( expression )

Built-in functions

print( string )

Writes a string to STDOUT.


Fetches a newline delimited string from STDIN.

call( identifier [, parameter])

Calls the function that is stored in 'identifier'.

tailcall( identifier [, parameter])

Performs a 'tailcall' to the function stored in 'identifier'. This type of call is used when recursing heavily to improve performance and to facilitate the use of functions as iterators. More information on tailcalls.

return([ value ])

Return a value from a function.

to_string( value )

Converts the given value to a representative string.

type_of( value )

Returns a string representing the type of the value given, eg: “integer”.

Reserved words

if, unless, function, end, call, tailcall, print, gets, return, to_string, type_of, nil, true, false


The default installation of Koi consists of the following discrete components:

Each of these components are completely stand-alone and are specifically designed to be as simple as possible. I strongly encourage pulling them apart to see how they tick, and changing them if you see fit! Just make sure to contribute any improvements back to the main repository!

Hope you enjoy Koi!

Author & Credits


Aaron Gough

Copyright © 2010 Aaron Gough (thingsaaronmade.com), released under the MIT license